Alumoxane is commonly used as an activator for metallocene catalyst components in the formation of metallocene catalyst systems. A well-known problem associated with alumoxane solutions is the formation of gel or gel-like particles in the solutions as they age. Typically the alumoxane solution will become more and more viscous until the solution completely gels. This typically occurs over a matter of weeks. More concentrated alumoxane solutions form gels more quickly.
The formation of gels in alumoxane solutions creates many serious problems. For example, it is difficult to transfer such solutions through a line. It is also difficult to effectively combine gelled alumoxane solutions with other substances and catalyst systems made with such solutions tend to have lower activity and have been shown to cause increased reactor fouling tendency when used in olefin polymerization.
Gel formation is believed to be associated with continued reaction leading to increased molecular weight of the oligomeric alumoxane structure. Many methods for reducing gel formation have been described. Refrigeration has been shown to reduce the rate of gel formation, however, refrigeration is quite costly. Vigorous shaking of a gelled alumoxane solution temporarily disperses the gels. Gels can partially be removed by decanting the solution. Unfortunately, these methods not only involve extra effort but are not entirely effective.
A number of methods for preparing gel-free or reduced gel alumoxane 5 solutions have been described. For example, U.S. Pat. No. 5,157,137 describes a method of making alumoxane wherein the alumoxane solution is treated with an anhydrous salt and/or hydroxide of an alkali or alkaline earth metal. It is reported that alumoxane solutions made in this way remain gel free for an extended period of time. U.S. Pat. No. 5,235,081 describes a method of removing gels from alumoxane by mixing an aromatic hydrocarbon solvent of the alumoxane with an aliphatic hydrocarbon solvent and then separating the precipitated solids from the alumoxane solution. U.S. Pat. No. 5,371,260 describes the use of primary or secondary amines with subsequent heat and filtration to prepare gel-free alumoxane solutions. U.S. Pat. No. 5,416,229 describes the use of certain electron-rich heteroatom compounds to stabilize alumoxane solutions and reduce gel formation. While these methods may be somewhat effective, it has been found that some catalyst systems made using such stabilized alumoxanes exhibit inferior performance.
The present inventors have found that by adding metallocene to an alumoxane solution, the alumoxane solution surprisingly remains gel free for an extended period of time. Such solutions may be successfully used to prepare catalyst systems that are highly active and that do not cause extensive fouling of the polymerization reactor when used to polymerize olefins.